1. Field of the Invention
This invention is directed to microwave antennas, in general, and to a two-axis antenna which can be stabilized relative to the feed assembly orientation, in particular.
2. Prior Art
There are many antenna systems known in the art. These antenna systems can be used in various information transmitting and/or receiving systems or the like and can be used for tracking and/or signalling. Most of the known antenna systems operate on a rotating basis to provide both the azimuth and elevation variable. This two-axis antenna system is usually arranged to be supported on bearings and driven by a motor gear-train apparatus. Thus, two degrees of rotation are achieved.
However, one problem that occurs in communications systems is that two or more datalink channels physically overlap and interfere with each other if more than one channel happens to be operating in the same geographical area. Clearly, it is quite desirable to isolate the interferring channels from each other. One method of isolation is using orthogonal linear polarization. This method works because orthogonal signals do not couple to each other.
In the past, many datalinks between antennas in a communications system have utilized circular polarization, inasmuch as this arrangement allows an airborne platform to maneuver without losing signal strength at the ground station. That is, the signal with circular polarization moves around; but does not tilt. Also, circular polarization provides some relief from multipath problems at low angles.
Nevertheless, in some applications orthogonal polarization is needed to counter the overlap problem. One approach in this regard is to use right-hand and left-hand circular polarization of signals because these signals are orthogonal to each other. Also, using dual linear polarization, with the individual linear polarizations at right angles to each other, produces signals with orthogonal polarization.
Of course, a problem with using circular polarization is that the circularity has to be devised and maintained extremely accurately. On the other hand, linear polarization requires the orientation of the two polarizations (for instance, vertical and horizontal) to be maintained very accurately with no tipping of the electromagnetic fields. That is, it must be recognized that the polarization of signals produced by airborne units which have a linear polarization will be tipped every time the airplane manuevers. More generally, in fact, tipping occurs almost any time that the antenna is moved and points in some other direction. Thus, it is required to devise some means to provide dual linear polarization wherein the polarization orientation can be maintained very accurately.
The desirability of increasing spectrum use efficiency of the data link arrangement by means of polarization isolation has been discussed recently. That is, utilization of two or more different signals on the same channel, but isolated from each other, increases the efficiency of the signal spectrum. Orthogonal polarization has long been used to provide the isolation between two signals on the same channel in the field of satellite communications (frequency reuse) and others.
It is also desirable to use polarization isolation for air-to-ground and air-to-air datalinks, but the polarization accuracy required for circular polarization is difficult to achieve for the airborne antenna. Because circular polarization is difficult to achieve, it would appear desirable to use linear polarization, and stabilize the polarization spatially. However, aircraft motion often causes rotation of linear polarization and results in cross-polarization coupling to the orthogonal channel. Therefore, what is needed is a means to stabilize the polarization axis as the aircraft maneuvers. The purpose of this invention is to accomplish this stabilization.